i want to hide certain values to hide in Multi-select pick list based on profile. i have tried validation rule for hide but it saying Error: Field__c is a multi-select pick list field. Multi-select pick list fields are only supported in certain functions.
As per my understanding you want to hide / show options within a multi picklist based on Profiles.
I believe you will need to use RecordTypes and assign the respective Picklist values to the appropriate Record Type. The Record Types are then assigned to Profiles.
Refer the following article, to understand how to Assign Record Types to Profiles in the Original Profile User Interface.